In a multipoint video conference, it is desirable that compressed video data generated by each participant should be made available to every other participant who wants to receive it. Over a unicast network, this requires repeated transmissions of the same data to several participants/endpoints, either by each endpoint source or by a multipoint control unit (MCU). An MCU receives data generated by all the participants/endpoints and sends the data, usually as a mix, to the other participants/endpoints who want to receive it. Clearly, the bandwidth requirements for a video conference on the unicast network system increases in proportion to the number of conference participants.
When a network-supported multicast is available for a video conference, each endpoint can send its video data only once into a multicast group. Other endpoints that want to receive the data can join the multicast group. The network can establish optimized distribution trees to transport the multicast video data to the members of the multicast group using well-established techniques (e.g., the DVMRP protocol as described in RFC 1075).
In a receiver-driven layered multicast technique (see e.g., Receiver-driven layered multicast, Steven McCanne, Van Jacobson, Martin Vetterli, ACM SIGCOMM Computer Communication Review, Volume 26, Issue 4, Pages 117-130, October 1996, ISBN:0-89791-790-1), an MCU is not required and video data are encoded using a layered or scalable coding technique where each additional layer received increases the quality of the received video. Each layer is then sent to a separate multicast group by each endpoint, allowing each receiver to choose the bandwidth and the reception quality it receives from other participants by selecting the particular multicast groups it joins.
Although use of multicast techniques can result in efficient use of the network bandwidth for multipoint conferencing, a conferencing system architecture that depends solely on network supported multicast, i.e., without a specialized MCU, has several shortcomings:                1. Network supported multicast is not available on the global Internet. Thus, a purely multicast-based solution can not be used for global multipoint conferencing.        2. When multicast groups are managed locally on different networks that are not multicast connected to each other, a mapping between these multicast groups must be established.        3. Multicast group address management for two or more simultaneous conferences must be carried out jointly to eliminate potential address conflicts.        4. When packet data is lost by a receiving endpoint, re-multicasting the lost data from the source to the entire group of participants/endpoints is not efficient because other member endpoints will receive redundant information.        5. When a new participant joins a multicast group, the compressed video may not be decodable for the new participant because predictive encoding may be used for the compressed video.        
Therefore, for efficient multipoint videoconferencing over partially or fully multicast supported networks, consideration is now being given to the design of a new multicast and scalable coding aware MCU or server.